The present invention relates to a lithium secondary battery using a non-aqueous liquid electrolyte containing lithium ions as migrating ions, the liquid electrolyte for the battery and use of the battery.
A lithium secondary battery using a non-aqueous liquid electrolyte has high potential and energy density and distinguished storage capacity and low temperature workability and has been widely used in consumer-oriented portable electrical products. Intensive researches and developments are now underway for battery scale-up for application to power storage systems during night hours for electric cars or domestic use.
However, most of solvents utilizable for such applications have a low flash point and a high flammability and thus have hazards of ignition, explosion, etc. due to overcharging, heating, etc. To assure the safety of the battery many approaches have been so far proposed.
For example, JP-A-7-192762 discloses mixing of a cyclic carbonate ester with flame retardant effect-expectable halogenated formate ester, thereby reducing the flammability. Furthermore, JP-A-8-45544 discloses mixing with a halogenated ester. However, the halogenated formate ester or halogenated ester has a lower flash point in some cases than non-halogenated cyclic carbonates and thus it seems that no satisfactory flame retardant effect can been attained thereby. JP-A-4-184370 and JP-A-8-88023 disclose another approach by adding a self-extinguishing effect-expectable phosphate ester to a liquid electrolyte, but the cycle characteristics are somewhat deteriorated thereby.
The foregoing approaches are examples of using a highly self-extinguishing solvent or an extinguishing solvent in a non-aqueous liquid electrolyte. The ultimate means of making a lithium secondary battery non-flammable is to use a per se non-flammable solvent of fluorine compound in the non-aqueous liquid electrolyte. As to the use of a fluorine compound, JP-A-9-293533 discloses addition of 0.5 to 30% by weight of a fluorinated alkane to a solvent, thereby making the solvent flame-retardant, where the fluorinated alkane acts as an extinguishing agent due to the suppressing effect of these low boiling point fluorine compounds on the flammability the resulting gas mixture. It cannot be said from the viewpoint of the mixing ratio that the disclosed liquid electrolyte is the one using a non-flammable solvent. Furthermore, JP-A-9-293533 discloses a flame retardation method comprising adding a fluorine compound having a boiling point of not more than 25.degree. C. to the battery, allowing the fluorine compound to evaporate faster than the flammable solvent when the battery is exposed to high temperatures, so that the flammability of flammable solvent can be suppressed by the resulting vapors, where the non-flammable solvent is used not in the liquid electrolyte, but as separated from the liquid electrolyte.